1. Field of the Invention
This invention relates to an optical recording medium.
2. Prior Art
Optical recording media include write-once disks using organic dyes as the recording material such as CD-R and DVD-R and rewritable disks using phase change recording materials such as CD-RW and DVD-RW. These optical recording media have recording layers formed on disk substrates which are provided on the surface with (guide) grooves for tracking and other purposes. Among these media, DVD-R and DVD-RW capable of higher density recording have address information-bearing prepits formed in raised regions (commonly referred to as lands) between adjacent grooves. Optical disks having prepits formed in lands are disclosed, for example, in JP-A 2000-11460 and 11-126372.
In optical recording media having grooves and prepits formed therein wherein the grooves serve as recording tracks, write/read operation is carried out along the grooves. It is believed that write/read characteristics are largely affected by the shape of the grooves and prepits.
The invention pertains to an optical recording disk of the structure comprising a substrate provided with grooves and lands and a recording layer containing an organic dye thereon, the grooves serving as recording tracks and the lands being formed with prepits. The disk substrate with grooves and lands is often prepared by the injection molding of resin using a stamper. The inventors attempted to prepare such disk substrates by changing many parameters including those associated with the stamper manufacturing step.
Now a common process for the preparation of a disk substrate is described. The disk substrate is prepared by the injection molding of resin using a stamper bearing a negative prepit and groove pattern. The stamper is often formed of nickel. In order to manufacture the stamper, an optical disk master is first prepared. The following process is generally employed for preparing the optical disk master. First, a photoresist layer is applied on the surface of a glass substrate. The photoresist layer is exposed to a laser beam to form a latent image of the desired pattern, followed by development.
Using the optical disk master, a stamper is manufactured usually by the following process. First, to impart electric conductivity to the surface of the photoresist layer on the optical disk master, a metal thin film of nickel or the like is formed as by sputtering or electroless plating. Electroforming is then effected to deposit a film of nickel or the like on the metal thin film. Then the laminate of the metal thin film and the electroformed film is stripped from the photoresist layer. The laminate is ready for use as the stamper master. This stamper master may be used as the stamper directly, although a stamper mother may be prepared from the stamper master and used as the stamper. The stamper mother is prepared by electroforming a film on the surface of the stamper master and stripping the electroformed film. It is recommended to previously oxidize the surface of the stamper master so that the electroformed film may be readily stripped therefrom. Alternatively, a stamper child may be similarly prepared using the stamper mother and used as the stamper.
In the process of preparing an optical disk master, the minimum width of the latent image pattern formed in the photoresist layer is limited by the diameter of a laser beam spot. The beam spot diameter w is represented by w=xcex/NA wherein xcex is the wavelength of the laser beam, NA is the numerical aperture of an objective lens in an optical system. In forming a latent image pattern corresponding to the groove, the photoresist layer is helically scanned with the laser beam.
When prepits are to be formed between grooves, two laser beams were used in the step of forming the latent image pattern. One laser beam is continuously irradiated to form a groove pattern and the other laser beam is intermittently irradiated to form a prepit pattern. The laser beam defines a beam spot having a Gaussian intensity distribution. In the event that the track pitch is narrowed in order to achieve a higher recording density, the perimeters of two laser beams overlap (that is, the skirts of two Gaussian distributions overlap) near the region which is simultaneously exposed to two laser beams, that is, the prepit-forming region. The dose of exposure can exceed the threshold in the overlapped area. The area where the dose of exposure has exceeded the threshold is removed upon development so that the land is lowered in height. A lowering of the land leads to increased errors.
To prevent the land from lowering its height, the size of the beam spot may be reduced in accordance with the distance between the groove and the prepit. This is achieved by either reducing the wavelength of a laser beam or increasing the numerical aperture of an objective lens in a laser beam recorder. As to the laser wavelength, a blue laser of short wavelength has been used in practice. Then, a laser beam of a shorter wavelength, for example, in the ultraviolet region must be used. On use of a laser beam in the ultraviolet region, however, optical components including the objective lens should be changed to those capable of complying with the ultraviolet region. This gives rise to a serious problem that a substantial modification of the system is needed.
An object of the invention is to provide an optical recording medium having grooves serving as recording tracks and prepits formed between adjacent grooves wherein the potential occurrence of errors due to an increase of recording density is minimized without a substantial cost increase.
The invention provides an optical recording medium comprising a substrate including lands and grooves extending substantially parallel on one surface thereof, the grooves serving as recording tracks, and the lands being formed with prepits. The height HP of the land disposed between the prepit and the groove and the height HL of the land in a prepit-free region, both as measured from the bottom of the groove in a cross section perpendicular to the substrate surface, satisfy the relationship: 0.25xe2x89xa6HP/HL less than 1.
In a preferred embodiment, the grooves are arranged at a pitch P, and the distance from the center of the prepit to the centerline of one adjacent groove is r, the pitch P and the distance r satisfying the relationship: 0.4xe2x89xa6r/Pxe2x89xa60.6.
The optical recording medium may further include a recording layer containing an organic dye on the substrate surface.